A. Field of the Invention
This invention relates to semiconductor devices and processes of manufacture. More particularly, the invention relates to semiconductor devices with reduced surface leakage paths and methods of manufacture.
B. Description of the Prior Art
The formation of surface leakage paths on a semiconductor wafer by inversion is a common problem that may occur in bipolar and field effect transistors. For example, an N-type silicon wafer with an oxide layer grown on the surface may invert to P-type conductivity through several physical mechanisms. One mechanism is the accumulation of a fixed negative charge in the oxide of sufficient quantity to invert the underlying silicon substrate from N to P-type. The presence of such an inversion layer between adjacent P regions in an N substrate gives rise to undesirable leakage currents between the regions. Another mechanism of inversion arises if conductors deposited on the oxide assume negative potentials in excess of the threshold voltage of the underlying N-type silicon substrate.
There are several known methods of preventing inversion in N-type silicon substrates. One method increases the oxide thickness. The thickened oxide increases the probability of metal conductors breaking at the point where they pass over a junction in the oxide. Etching problems are also created by the thicker oxide which requires generous layout dimensions. A second mechanism increases the donor concentration in the potential inversion regions. The increased donor concentration, however, lowers the junction breakdown voltage for adjacent P-type regions. A third method inserts N+ channel stoppers within the inversion regions. Channel stoppers are widely used but require additional layout area. They also impede design automation because the locations of inversion regions are not known until the layout is complete. A fourth method of preventing inversion is to implant positive charges into the oxide film. In Applied Physics Letters 19, Dec. 1971, pgs. 478-479, G. Sixt and A. Goetzberger have demonstrated that cesium ions implanted into the silicon surface prior to oxidation provides a stable positive oxide charge. However, the data described by Sixt and Goetzberger indicates that the positive ion charge becomes less effective as the oxide grows thicker, probably because the cesium ions prefer to concentrate near the oxide surface and/or are lost by evaporation during oxide growth.
Semiconductor structures and processes that minimize surface leakage paths between adjacent devices without (1) increasing reliability risks, (2) degrading device parameters, (3) requiring the use of specilized implantation apparatus, or (4) imposing the need for additional layout area would improve the present state of technology.